BIOLOGICAL MACROMOLECULES
POLYSACCHARIDES AND PROTEINS
| Contents for this page | Related topics |  |
|---|---|---|
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Introduction Polysaccharides Polypeptides and proteins Additional questions |
Nucleic acids | |
| Learning Outcomes | ||
| After studying this section, you will understand what is meant by substitution, addition, and elimination reactions | ||
Living organisms are utterly dependent on various macromolecules. These are of various types, and serve numerous functions, most of which are outside the scope of this course, but some of which are illustrated in the table below:
| Polymer type | Functions | Example |
|---|---|---|
| Polysaccharides | Structural Energy storage |
Cellulose (plants) Chitin (insects) Starch (plants) Glycogen (vertebrates) |
| Proteins | Structural Catalysts Carriers Nutritional sources for amino acids |
Collagen Enzymes Hemoglobin All |
| Nucleic acids | Information processing | DNA, RNA |
| Lipids | Energy storage Structural |
Fats, oils Nerve sheaths |
The science of biological macromolecules and the relationship between their structures and biological functions is called STRUCTURAL BIOLOGY.
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POLYSACCHARIDES are condensation polymers of simpler units called MONOSACCHARIDES, which belong to a very large group of organic molecules called CARBOHYDRATES, that is, molecules that contain only carbon, hydrogen and oxygen ( ). A simple monosaccharide, glucose, has the structure shown here on the left. |
Thus, cellulose, the structural material in plants, and starch, a storage polysaccharides in plants may be considered to be condensation polymers of glucose with a subtle difference in structure (perhaps you can spot it? ):
Polypeptides and proteins are condensation polymers of organic substances called a-AMINO ACIDS, which have the general structure shown on the right (top). In the crystalline form and in solution, these amino acids are best represented as the ionised molecule (right, bottom) ( |
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Natural polypeptides and proteins are made up of 20 different amino acids, differing in the nature of the group R (). The structural formulae and names of these amino acids are given elsewhere. The polymeric nature of these compounds are derived from the condensation of amino and carboxyl groups to form a strong, covalent PEPTIDE BOND. Simply put, the formation of a peptide bond can be visualised as the condensation between two amino acids (shown below as two different amino acids with groups R1 and R2) with formation of a water molecule. Bear in mind however that this is NOT how the bond in formed by biological systems!
In a polypeptide or protein, the 20 amino acids are arranged in a seemingly random order. The number of possible combinations is truly astronomical. For a small protein containing only 100 amino acids, each one picked at random from the collection of 20 amino acids normally found in proteins, 20100 (this equals 1.27x10130) different protein molecules can be constructed! The precise order of the amino acids in the polymer chain can be determined in the laboratory, and is known as the PRIMARY STRUCTURE of that peptide, polypeptide or protein. In living organisms, very large numbers of proteins of various sizes interact to make life possible. Each of these proteins has a specific primary structure which is determined genetically, as we will see in the section on DNA.
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Many polypeptides consist of several chains, either joined covalently by -S-S bonds, or by combinations of hydrogen bonds, ionic bonds, and other non-covalent forces. The sequence of human insulin, a polypeptide hormone, shown on the left, is made up of two chains joined by three covalent -S-S- bonds. The structure of polypeptides and proteins extends in three dimensions, as the various interactions between the amino acid side-chains cause specific and intricate folding, normally absolutely essential for biological activity. On the right, below, we have a picture of the single chain of myoglobin, an oxygen-carrying protein found in the muscles of vertebrates. Note the presence of an Fe atom. |
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On the right, above, we see a picture depicting the three-dimensional structure of hemoglobin, the oxygen-carrier in red blood cells. This molecule is made up of four chains, each very similar but not identical with myoglobin, and each of which are also bound to an iron atom.
The table below gives the names and abbreviations of the 20 amino acids found in proteins, together with the structures of the side chain group R, except for histidine and proline, where the whole amino acid structure is given. |
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| Name | Abbreviation | One-letter abbreviation | Structure of R |
|---|---|---|---|
| Alanine | ala | A | -CH3 |
| Cysteine | cys | C | -CH2SH |
| Aspartic acid | asp | D | -CH2COO- |
| Glutamic acid | glu | E | -CH2CH2COO- |
| Phenylalanine | phe | F |  |
| Glycine | gly | G | -H |
| Histidine | his | H |  |
| Isoleucine | Ile | I | --CH(CH3)CH2CH3 |
| Lysine | lys | K | -CH2CH2CH2CH2CH2NH3+ |
| Leucine | leu | L | -CH2CH(CH3)CH3 |
| Methionine | met | M | -CH2CH2S-CH3 |
| Asparagine | asp | N | -CH2CONH2 |
| Proline | pro | P |  |
| Glutamine | gln | Q | -CH2CH2CONH2 |
| Arginine | arg | R |  |
| Serine | ser | S | -CH2OH |
| Threonine | thr | T | -CH(OH)CH3 |
| Valine | val | V | -CH(CH3)CH3 |
| Tryptophan | trp | W |  |
| Tyrosine | tyr | Y |  |
